Mechanical springs are widely used to serve a variety of functions, such as exerting a resilient force, providing flexibility, and storing and absorbing energy. One type of mechanical spring that has found many uses is a torsion bar.
In general, torsion bars are straight bars made of inherently resilient or elastic material, and designed to be subjected to torsional loading, i.e. twisting about its longitudinal axis. The torsion bar is usually solid and circular in cross-sectional configuration, but may also be of square or rectangular cross-section. Torsion bars have many applications; one well-known use being in automotive vehicle suspension systems.
Historically, torsion bars have been fabricated out of metal, such as steel. This provides the desired strength and durability, but does not provide the ideal resiliency due to its high modulus of elasticity. As the cost of component parts for automotive vehicles continues to rise and the need to save weight to meet governmental standards for improved gas mileage increases, the need arises to design a less expensive and lighter torsion bar. To compliment this goal, designers are continuing to create a torsion bar fabricated of a material with a low modulus of elasticity. A material found to meet the dual requirements of providing high elasticity and sufficient strength is a composite material made of resin coated glass fibers. The use of this material provides a lightweight product that is cost effective. A further advantage is that such a composite material alleviates the critical availability of metal alloys.
A problem has always existed in connecting torsion bars to any mass to be sprung due to the significant stresses existing at the connection regions. Thus, there is a need to improve the connection used in incorporating the new generation torsion bar fabricated of glass fiber/epoxy composite material into the suspension system of a vehicle. Most of the operating components of a vehicle suspension system are fabricated of metal to provide strength. It is thus anticipated that opposing ends of a torsion bar would cooperate with metal components of significant strength and mass.
It is recognized that the desirable elasticity characteristic of a torsion bar providing the amount of deflection necessary to respond to vehicle motion conflicts with the strength requirement needed for use as attachments to the vehicle suspension system. The attachment component requires a high modulus of elasticity to introduce strength to the connection. Strength is inherently reduced when a torsion bar is required to have the necessary elasticity to perform its function.
One approach used in the art involves securing a torsion bar fabricated of composite material directly to a metal mounting piece. The torsion bar normally has a tubular configuration and is fitted into a tubular socket in the anchor. This type of direct attachment produces major problems since the torsion bar fabricated of composite material tends to deflect much more than the metal mounting piece when torque is applied, resulting in the tendency for a shear failure at the attachment interface.
U.S. Pat. No. 3,856,289 to Steele discloses an attachment design between a torsion bar fabricated of fiberglass and a metal anchor attempting to remedy the problem. Two tapered elastomeric annuli in the anchor body permit angular deflection of the torsion bar relative to the anchor. This design reduces the stress concentration at the plane of entry of the torsion bar in the anchor, thus reducing the chance of failure of the torsion bar.
This attachment design favorably allows the increased use of torsion bars fabricated of composite material offering the cost and weight advantages described above. The use of elastomeric securing means between the torsion bar and the metal mounting piece greatly reduces the failure rate by reducing the stress concentration at the attachment interface. This results in turn in a greatly increased operating life. This improved situation increases the incentive to substitute more elastic and resilient torsion bars fabricated of composite material in place of metal torsion bars, decreasing the weight of the vehicle and also requiring less space in most cases.
A further improvement in the design of torsion bars for automotive vehicle suspension systems and other uses has involved using a bundle of resin coated glass fibers formed into a rod-like shape. Several rod-shaped fiber bundles are then twisted together and set in to a rope-like configuration. This produces a twisted rope torsion bar fabricated of lightweight glass fiber/epoxy composite material incorporating the benefits of increased elasticity and resilience offered by the composite material with the higher load capacity offered by the twisted rope configuration. It should be appreciated that the attachment concept disclosed in the '289 patent is not ideally suited for use with twisted rope torsion bars. The benefits offered by the elastomeric annuli of the previous design derive from the circular orientation which provides a firm grip of the torsion bar at all points of engagement. An improved attachment design is thus needed to allow the twisted rope torsion bar to optimally cooperate with a metal mounting piece and at the same time maintain the reduced stress levels at the attachment interface.